As packaging films, there are generally used oriented or stretched films of polypropylene and polyethylene terephthalate. However, the polypropylene film is poor in oxygen gas barrier characteristics, while the polyethylene terephthalate film is poor in moisture resistance, i.e., water vapor barrier properties.
There is also generally used a composite film obtainable by laminating polypropylene films on both surfaces of a film of a saponified ethylene-vinyl acetate copolymer through adhesive layers containing maleic anhydride modified polypropylene. In such a composite film, however, the oxygen gas barrier characteristics of the film of the saponified ethylene-vinyl acetate copolymer are considerably decreased when the film absorbs moisture. For example, the oxygen gas barrier characteristics are decreased by about 10 to 40 times after a treatment of retort sterilization. This makes it difficult to preserve foods or the like for a long period of time with such a composite film. Further, due to the polymer components, this composite film is poor in heat resistance and produces a smell of potyolefin particularly when the film is heated.
There is also known a composite film obtainable by laminating polypropylene films on both surfaces of a film of polyvinylidene chlorides through layers of an ethylene-vinyl acetate copolymer. However, such a composite film is decreased in gas barrier characteristics due to a plasticizer or a stabilizer so added as to impart a molding ability to the polyvinylidene chloride polymers. Further, since the glass transition temperature of the polyvinylidene chloride polymers is low, the polyvinylidene chloride polymers are apt to be plasticized as the temperature is raised. This decreases the gas barrier characteristics of this composite film at a high temperature. Further, this composite film is poor in heat resistance due to the polymer components.
As a film excellent in heat resistance, a polyethylene terephthalate film is known. Accordingly, it may be proposed to co-extrude such a polyethylene terephthalate film and the polyvinylidene chloride film above-mentioned to form a laminated film. In the polyvinylidene chloride polymer, the melting point is located in the vicinity of the decomposition temperature, and the polyvinylidene chloride polymer is decomposed at a temperature from about 180 to about 200.degree. C. It is therefore difficult to make a film by co-extending and laminating, by a molding method, the polyvinylidene chloride polymer and polyethylene terephthalate excellent in heat resistance. Even though the polyvinylidene chloride polymer layer and polyethylene terephthalate layer are laminated on each other, heat sealing properties may not be assured. Accordingly, the resultant film may be used to considerably limited applications only.
As thus described, the packaging films are considerably different from one another in the characteristics of polymer components thereof. It is therefore difficult that a one-type film simultaneously satisfies a plurality of characteristics. Accordingly, a variety of packaging films are respectively used according to the purposes.
A liquid crystal polymer classified in engineering plastics is excellent in mechanical properties, dimensional stability, heat resistance, chemical stability and electric properties. Thus, attention is given to the liquid crystal polymer as a polymer film satisfying a variety of requirements. Japanese Patent Publication No. 42287/1985 describes that the liquid crystal polymer film may be divided to form highly elastic split fibers. As apparent from this description, such a film tends to orient in the polymer flowing direction, and is extremely weak in strength in the widthwise direction (hereinafter referred to as TD direction) at a right angle to the film drawing direction (hereinafter referred to as MD direction). To improve the strength of the liquid crystal polymer film in the TD direction, Japanese Patent Application Laid-Open No. 46728/1981 and No. 102234/1986 disclose a method of preparing a biaxially oriented or stretched film by increasing the blow ratio at the time when the film is molded by an inflation method. Japanese Patent Application Laid-Open No. 2127/1981 and No. 173620/1988 disclose a method in which a ring die is rotated at the time when a liquid crystal polymer film is molded by an inflation method. Japanese Patent Application Laid-Open No. 59818/1983 discloses a method in which a porous plate-like member having a plurality of fine gaps which can be heated, is formed inside a T-die. Further, Japanese Patent Application Laid-Open No. 31729/1988 discloses a method using a three-layer co-extrusion die, by which both external layers outside of the intermediate layer of a liquid crystal polymer are formed by thermoplastic polymers which are not adhesive to the liquid crystal polymer, and, after the three layers have been coextruded, the external layers are separated, thereby to obtain the liquid crystal polymer film of the intermediate layer.
However, the liquid crystal polymer films thus obtained are still weak in strength in the TD direction with respect to the MD direction, and are large in mechanical anisotropic properties. Likewise polyethylene terephthalate, the liquid crystal polymer film does not have sufficient thermal adhesive properties. It is therefore not possible to pack an article with such a film by heat seal. Thus, the application of such a film is considerably limited.
It is a main object of the present invention to provide a composite film capable of satisfying a variety of requirements.
It is another object of the present invention to provide a composite film containing, as an element, a liquid crystal polymer layer, yet assuring excellent heat adhesive properties.
It is a further object of the present invention to provide a composite film not only assuring excellent gas barrier characteristics even though exposed to an atmosphere of a high temperature and a high humidity, but also having heat sealing properties.
It is still another object of the present invention to provide a composite film excellent in heat resistance which produces no smell of polyolefin.
It is still the further object of the present invention to provide a composite film containing a liquid crystal polymer layer improved extremely in the mechanical anisotropic properties.